Biosensors are essential in preventing epidemics for public and global health, warning of intentionally released agents for national security and defense, and fundamental biology and pharmacology research for early disease detection and drug discovery. These applications require biosensors that possess several critical properties for reliable and rapid detection. For instance, label-free biosensors can eliminate problems associated with labelling steps. Biosensors with ultra-sensitive optical responses can accurately distinguish minute changes in molecular level. Ability to operate in real-time can enable analysis of biomolecular binding kinetics. Massively multiplexed biosensors can allow parallel screening of large variety of biological assays. Portable biosensors that are easy-to-operate in a cost effective manner can be used in resource-poor settings. Recently, plasmonic biosensors utilizing nanoparticle and nanoaperture geometries have received significant attention as they can meet these needs.
In particular, nanohole arrays fabricated on optically thick metal films are highly promising. These subwavelength apertures enable extraordinary optical transmission (EOT) phenomenon due to the effective excitation of plasmons at normal incidence by grating coupling. This feature allows compact biosensors by eliminating the bulky prism-coupling mechanism needed by conventional surface plasmon resonance (SPR) sensors. Even though SPR schemes have very sensitive response of around 10-7 RIU (refractive index unit), their angle-sensitive optical setup limits large-area multiplexing and high-throughput biodetection. Plasmonic modes supported by nanohole arrays are highly sensitive to surface conditions due to their strong field enhancements and light confinement in nanometer scale. Consequently, local refractive index changes induced by the binding of minute quantities of biomolecules on the sensor surface can be detected by monitoring the spectral variations within the plasmonic modes without any need for fluorescent labels.
Nanohole arrays are also compatible with imaging-based devices and can be implemented in a microarray format for multiplexed and high-throughput biosensing. The optical extinction settings on collection of nanohole transmission could be implemented in optical settings that are cost-effective and portable. Recently, plasmonic nanoholes have been utilized in a lens free microscope with a normally incident light-emitting-diode (LED) source and a complementary metal-oxide semiconductor (CMOS) camera to demonstrate a low-cost handheld biosensor for resource-poor and field settings. Integrating with microfluidic systems, nanohole biosensors also enable real-time analysis of biomolecular binding kinetics.
As discussed above, plasmonic nanohole arrays have received significant attention as they have highly advantageous optical properties for ultra-sensitive and label-free biosensing applications. However, these subwavelength periodic apertures are mainly implemented on transparent materials, which results in multiple spectrally close transmission resonances. However, this spectral characteristic is not ideal for biosensing applications as it complicates monitoring spectral variations. In light of these and other deficiencies in the field of biosensing and the use of nanohole arrays, new and superior solutions are desired.